Method and device for the colorimetric measurement of a colored surface

ABSTRACT

The present invention relates to a method for the colorimetric measurement of a defined region on an image representing a surface, characterized in that the said image of the said surface is taken using a color camera, in that the analogue information measured on the said surface is transformed and converted into digital form, in that the said defined region on the said image is delimited and in that the measurement of the chromatic values R, G, B is carried out for this region.

SUBJECT OF THE INVENTION

The present invention relates first of all to a process which allows thecolorimetric and possibly dimensional measurement of a coloured surfaceand to the processing of the information measured on the said surface oron part of the latter.

The present invention also relates to the device for the calorimetricmeasurement of the said surface.

PRIOR ART

Many calorimetric measurement-taking devices are known which make itpossible to give, by tristimulus measurement, values of the threechromatic parameters, R (red), G (green) and B (blue), of a calorimetricmeasurement. For this purpose, an illuminant is used so as to illuminatea surface on which the measurement will be taken. It should be pointedout that this measurement is normally a measurement of the averagechromatic values of the illuminated surface.

Patent Application FR-A-2,749,077 has proposed a method and a device formeasuring the colour by calculating the trichromatic components withrespect to a colorimetric reference system. This calculation is madeusing a matrix for transferring the calorimetric system associated withthe data acquisition system to the calorimetric reference system. Thesaid transfer matrix is calculated by an iterative procedure based onthe trichromatic components measured by the calorimetric referencesystem of the three primary colours. However, it is found that themeasurements are taken using a tri-CCD camera having three CCDsensors—one CCD sensor for each of the three trichromatic components.The cost of this camera is relatively high. Furthermore, it is foundthat the illuminant is placed outside the acquisition system includingthe camera. This means that perturbations due to the surrounding(ambient) illumination will be picked up during measurement by theacquisition system. Furthermore, it is found that no dimensional orspatial processing of the image is possible with the system described inthis publication.

Document U.S. Pat. No. 5,724,259 describes a method for monitoring thecolour of a printed image on a substrate, which makes it possible, usinga camera, to measure the light reflected from the said printed imageonto the substrate, to transform this measurement into a signal sent toa computer and to use the information emanating from this signal tocorrect the effects of scattered light reflected from the printed image.The main use of this method is therefore to check the ink thickness inthe case of applications of printing machines such as printers. For thispurpose, the optical density defined by the logarithm to the base 10 ofthe reflectance, this being a percentage of the scattering of theincident light, is more specifically measured. For this purpose, areference surface not incorporated into the apparatus is placed besidethe printed image to be measured. This also means that the measurementis not a contact measurement but indeed a remote measurement.

Document EP-A-0,491,131 describes an apparatus for monitoring andcalibrating the spectrum emitted by a coloured object using thedispersion of the wavelength provided by a variable filter. This is acalibrating apparatus working directly as a calorimeter. However, in thepresent case, because of the presence of a diffuser and a variablefilter between the one-dimensional sensor and the object itself, it isestimated that no spatial information could be obtained by the use ofsuch a device.

Document EP-A-0,314,312 describes a method and an apparatus making itpossible to detect the presence of ink on a substrate using severallight-emitting diodes (LEDs) of different wavelengths which are switchedon and off independently of each other.

Document U.S. Pat. No. 3,597,094 describes a portable device allowingcolour identification, which works directly with visual observation.This device comprises a plurality of filters positioned between a lightsource and the coloured surface.

OBJECTS OF THE INVENTION

One object of the present invention is to propose an improved method andan improved device which make it possible to obtain a calorimetric anddimensional (or spatial) measurement of a defined region on an image ofa surface, which is possibly inhomogeneous, such as dot-matrix printingor a defect appearing on a captured surface.

This will allow a reproducible and reliable measurement of the chromaticparameters of the said defined region to be obtained.

The object of the present invention is also to propose a device and amethod which make it possible to take stabilized measurements withrespect to a reference standard.

The object of the present invention is also to allow the use of thisinformation for medical, industrial, cosmetic and food applications orelse in any other application using the technique of colorimetry.

MAIN CHARACTERISTIC ELEMENTS OF THE INVENTION

The present invention relates first of all to a method for thecalorimetric and possibly dimensional measurement of a defined region onan image representing a surface. The method consists in taking an imageof the said surface using a colour camera, in transforming andconverting the analogue information measured on the said surface intodigital form, preferably using a processing unit on board of the colourcamera or in a computer, in delimiting the said defined region on thesaid image and in carrying out the measurement of the chromatic valuesR, G, B for this region.

The term “surface” should be understood to mean the surface for which itis desired to obtain colorimetric-type and/or dimensional-typeinformation.

The term “image” should be understood to mean a group of pixels whichrepresents the captured surface and which may be directly transmitted toa monitor such as a screen or a printer.

The term “region” should be understood to mean that part of the saidimage for which the calorimetric and/or dimensional measurement iscarried out.

Optionally, the method also consists in measuring the distance betweentwo pixels of the said image or in delimiting the included area duringperipheral tracing of a region of the said image.

The definition of the said region may be carried out either manually byperipheral tracing, or automatically by scanning the various pixels ofthe image for which the chromatic parameters fall within a predeterminedwindow or interval.

The present invention also relates to the device for implementing thesaid method, comprising a sensor provided with a colour camera and withan illuminant which are placed within the same enclosure associated withan analogue information acquisition system, the information beingmeasured using the said sensor and being converted into digital form bya processing unit.

Preferably, the colour camera is a mono-CCD or mono-CMOS camera.

Preferably, the illuminant will be produced using a single, white orcolour, diode or several identical, white or colour, diodes having thesame spectral distribution.

According to a preferred embodiment, the sensor furthermore includes,within the same enclosure of the sensor, a reference standard placed inthe field of view of the said camera. This makes it possible tostabilize each measurement.

According to a preferred embodiment, the information converted by theprocessing unit is transmitted to a monitor which may be a screen or aprinter and which gives a precise display of the image. Optionally,using this monitor, the image may also be processed so as to measure thedistance between two specific points or pixels, to measure the area ofthe said region, etc.

The data will be obtained by a tristimulus measurement of the R, G, Bparameters, preferably by matrix computation.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an overall schematic view of the information acquisitionsystem according to the invention.

FIG. 2 shows an exploded view of the measurement sensor used in theacquisition system shown in FIG. 1.

FIG. 3 shows the flow chart for the processing method for themeasurement carried out on the image to be analyzed.

DETAILED DESCRIPTION OF ONE EMBODIMENT OF THE PRESENT INVENTION

FIG. 1 shows an overall schematic view of the acquisition system ofanalogue information relating to the measured chromatic values. Theanalogue information will be converted into digital form by theelectronics. This acquisition system firstly comprises a sensor 100which will be described in greater detail below and a processing unit200 which processes the information captured by the camera and transmitsthem to a monitor 300. The processing unit is composed of a camera card210 and a CPU card 220. The information is transmitted between the twocards digitally. The measurement head or sensor 100 is linked by alow-voltage flex to the camera card. This sensor essentially comprises,held within a single enclosure, a colour camera 150, preferably amono-CCD or mono-CMOS camera, and an illuminant 120 which may consist oflight-emitting diodes, these being, for example, white or other colours.The fact of integrating the illuminant into the actual measurement headmakes it possible to get round the problem of environmentalperturbations, given that the camera will be sensitive only to the imageilluminated by the illuminant and not to the external (ambient)environment.

FIG. 2 shows this sensor in greater detail. Conventionally, it comprisesan optional window 1 which may also protect the camera and the sourcefrom the external environment. The sensor will be placed directly on thesurface of the specimen to be analyzed. Provision is made for the latterto be at the object distance from the objective, thus allowing a sharpimage to be transmitted to the screen. The illuminant preferablyconsists of diodes geometrically integrated into the sensor in order togenerate, possibly using a diffuser 6, light which is perfectlyhomogeneous over the entire surface to be measured. Homogenization ofthe illuminant may furthermore be achieved using an integrating sphere.The stability of the illuminant is guaranteed by controlling the currentthrough the diodes. The optical barrel 3 of the camera is provided witha combination of lenses as well as with a diaphragm of well-defineddimensions, which allow the resolution of the sensor of the CCD colourcamera 5 to be fully exploited.

Particularly advantageously, provision is also made to place, possiblyin the internal field of view of the camera, such as for example on theinternal or external face of the window, a well-defined colour referencestandard so that the values measured while taking a measurement canalways be measured with reference to the reference standard 7 duringeach measurement. Thus, the data obtained, including for the referencestandard, will make it possible, at each measurement taken, tostabilize, that is to say to determine and evaluate, any drift in thesensor assembly and to take this into account during processing of theinformation and of the measured chromatic values.

The measurement is based on the tristimulus (R, G, B) method alreadymentioned above, the stability of which is ensured by the use of astable reference surface. The interpretation of the measurements isperformed, as required, in the various chromatic coordinate systems(xyY, Lab, etc.), depending on the choice of the user. The calculationof the co-ordinates is mainly performed by matrix calculation based onthe RGB chromatic values measured by the sensor. Absolute or relativemeasurements may be obtained, for example by measuring the deviationfrom one colour with respect to another. The precision of themeasurement will depend on the number of pixels selected (and thereforeon the number of bits per pixel) for the measurement, possibly allowinginterpretation of the image obtained on the screen of the monitor.

Particularly advantageously, not only the average chromatic valuemeasured for a group of pixels but also the entire image may thus betransmitted, which may be stored, compared or transmitted to theoutside. This technique furthermore allows very precise positioning ofthe sensor as well as repeated repositioning. It is found that themeasurement time is of the order of one second and that the chromaticvalues are displayed almost instantaneously on a display and possiblysimultaneously on the monitor interpreting the image.

FIG. 3 shows the flow chart for the measurement. A surface Sm to bemeasured, illuminated by a source S, gives an image I exploitable by thesensor.

The colour camera possesses three types of pixels, representative ofred, green and blue. The analogue information per pixel Ra(i, j), Ga(i,j), Ba(i, j) (for all the pixels) will be converted into digital form bythe electronics E into Rn(i, j), Gn(i, j), Bn(i, j) (for all thepixels). In order to correct the imperfection in the homogeneity of thesensor and of the illuminant, an image correction function Fc gives,from Rn(i, j), Gn(i, j), Bn(i, j) (for all the pixels), corrected valuesRc(i, j), Gc(i, j), Bc(i, j) (for all the pixels). This correctionfunction per pixel is established by placing the sensor on homogeneoussurfaces. These corrected values are then transferred by a videotransfer function Fv in order to be able to display the image of Rv (i,j), Gv(i, j), Bv(i, j) (for all the pixels) which is closest to reality,possibly taking into account the response of the monitor.

From the corrected values Rc(i, j), Gc(i, j), Bc(i, j) (for all thepixels), a reference region Rr(i, j), Gr(i, j), Br(i, j) (composed of agroup of pixels Npr) is imposed and another measurement region Rm(i, j),Gm(i, j), Bm(i, j) (composed of a group of pixels Np) is chosen using aselection function Fsel. The number of pixels Np may be obtained fromRm(i, j), Gm(i, j), Bm(i, j) and from a dimensional measurement functionFd, making it possible to obtain distance or area metrology M. Thesemeasurements may be displayed on the monitor.

Being in possession of the reference Rr(i, j), Gr(i, j), Br(i, j) (for agroup of pixels Npr), it is possible, using an integration function Fi,to find the values Rr, Gr, Br. The measurement of Rm(i, j), Gm(i, j),Bm(i, j) for a number of pixels Np gives Rm, Gm, Bm by integration.

With Rr, Gr, Br and Rm, Gm, Bm, it is possible to use a stabilizationfunction Fstab (which eliminates the influence of the sensor on thetemperature, etc.), giving R, G, B specific to the system.

In order to be able to give the chromatic values in a CIE referencesystem XYZ, a transfer function Ft is used, which gives XYZ from RGB.This XYZ system is not the only reference system. Consequently, by meansof a calorimetric function Fchr, is possible to obtain and display themeasured colour CM in xyY, Lab, LCH, etc.

Several peripheral devices can be used to transfer this data, such as aconnection to another computer, to a printer, to a network, etc.

What is claimed is:
 1. A method for a calorimetric measurement of adefined region on an image representing a surface, comprising:illuminating the region by using a sensor comprising an illuminant;capturing an image of the region with a colour camera which is presentin the sensor; capturing an image of a reference area which is in afield of view of the colour camera, the reference area being present onthe sensor; defining the region on the image during manual selection byperipheral tracing; and converting analogue information characterizingthe image into digital form, thereby stabilising the image of the regionby using the reference information.
 2. A method for a colorimetricmeasurement of a defined region on an image representing a surface,comprising: illuminating the region by using a sensor comprising anilluminant; capturing an image of the region with a colour camera whichis present in the sensor; capturing an image of a reference area whichis in a field of view of the colour camera, the reference area beingpresent on the sensor; defining the region on the image automatically byscanning the pixels which have chromatic values lying within apredetermined interval; and converting analogue informationcharacterizing the image into digital form, thereby stabilising theimage of the region by using the reference information.
 3. The method ofclaim 1 or 2, wherein a dimensional measurement of the defined region iscarried out.
 4. A method for a colorimetric measurement of a definedregion on an image representing a surface, comprising: illuminating theregion; capturing an image of the region with a colour camera; capturingan image of a reference which is not on the surface; and convertinganalogue information characterizing the image into digital form, therebystabilising the image of the region by using the reference information,wherein the region is defined during manual selection by peripheraltracing.
 5. An analogue information acquisition system for implementingthe method according to claim 1, comprising in combination: a sensor,comprising an enclosure, which includes at least a camera, anilluminant, and a reference of defined colour placed in the field ofview of the camera, the reference on a window which allows an internalpart of the sensor to be protected; and a processing unit, forconverting the image measured by the sensor into digital form and formaking it possible for the defined region, the chromatic parameters ofwhich it is desired to know, to be precisely delimited; and an outputdevice.
 6. The system of claim 5, wherein the camera is a CCD colourcamera or a CMOS colour camera.
 7. The system of claim 5, wherein theilluminant is produced using at least one light-emitting diode, whereineach of the at least one light-emitting diodes is identical and has thesame spectral distribution, and wherein one of the at least one lightemitting diodes is a white or colour light-emitting diode.
 8. Ananalogue information acquisition system for implementing a method forthe colorimetric measurement of a defined region on an imagerepresenting a surface, comprising the steps of: illuminating theregion; capturing an image of the region with a colour camera; capturingan image of a reference which is not on the surface; and converting theanalogue information characterizing the image into digital form, therebystabilising the image of the region by using the reference information,comprising in combination: a sensor, comprising an enclosure, whichincludes at least a camera, an illuminant, and a reference of definedcolour placed in the field of view of the camera; a processing unit, forconverting the image measured by the sensor into digital form and formaking it possible for the defined region, the chromatic parameters ofwhich it is desired to know, to be precisely delimited; and an outputdevice, wherein the reference is placed on a window that allows theinternal part of the sensor to be protected.
 9. A method forcolorimetric measurement of a defined region on an image representing asurface, the method comprising: illuminating at least a portion of thesurface using a sensor device comprising an illuminant; obtaining animage of at least a portion of the surface using a colour camera whichis present in the sensor device; transforming and converting at least aportion of analogue information in the image into digital form;selecting a region on the image; deriving from the image obtained by thecolour camera, a set of colorimetric values (Rm, Gm, Bm), of pointswithin the region; and imposing a reference area and obtaining a set ofcolorimetric values (Rr, Gr, Br) of points within the reference area,the reference area being present in a field of view of the colourcamera, the reference area being placed on the sensor device.
 10. Methodaccording to claim 9, further comprising the step of performing astabilization of the colorimetric values of the region by applying astabilization function using as input to the function the calorimetricvalues (Rm, Gm, Bm) of the region, as well as the reference values (Rr,Gr, Br) and obtaining as output of the function a set of stabilizedcalorimetric values (R, G, B) of the region.
 11. Method according toclaim 9, wherein a dimensional measurement of the defined region isperformed.
 12. Method according to claim 9, wherein the region isdefined during manual selection by peripheral tracing.
 13. Methodaccording to claim 9, wherein the region to be measured is selectedautomatically by scanning pixels which have chromatic values within apredetermined interval.
 14. Device for implementing the method accordingto claim 9 comprising: a sensor comprising an enclosure which includes acamera, an illuminant, and a reference of a defined colour, thereference placed in the field of view of the camera; a processing unitfor converting the image measured by the sensor into digital form, theprocessing unit for delimiting the region and obtaining chromaticparameters; and an output device.
 15. Device according to claim 14,further comprising means for performing stabilization of colorimetricvalues of a selected region, by applying a stabilization function, usingas input to the function values (Rm, Gm, Bm) as well as a set ofreference values (Rr, Gr, Br) and obtaining as output of the function aset of stabilized calorimetric values (R, G, B).
 16. Device according toclaim 14, characterized in that the camera is a CCD colour camera or aCMOS colour camera.
 17. Device according to claim 14, characterized inthat the illuminant is produced using a single, white or colour,light-emitting diode or using a plurality of identical, white or colour,light-emitting diodes having the same spectral distribution.
 18. Deviceaccording to claim 14, characterized in that the reference is placed ona window which allows an internal part of the sensor to be protected.